Electrochemical gradients concepts

The concept of a high-energy phosphorylated intermediate at coupling sites" was a feature of all earlier concepts of the mechanism of ADP phosphorylation. The possibility of such a precursor, engendered by the electrochemical gradient on Fi, has however been eliminated by studies involving the use of, sO-labeled components of the reaction ... 

Fig. 10.1. The thermogenin concept. In the model for mitochondrial thermogenesis presented here, the thermogenesis is assumed to originate from the action of the brown fat-specific protein, thermogenin. Thermogenin acts as an OH conductor, regulated by cytosolic nucleotides (here shown as ATP) and by the so-called mediator (see Section 5). The OH neutralizes the H electrochemical gradient created by respiration, and substrate oxidation occurs unhampered by this gradient and without energy conservation. (Adapted from Ref. 6.)...

The utilization of energy to drive electrons against an electrochemical gradient in biological systems has been termed reversed electron transfer and has been an intensely debated topic for more than twenty-five years. The concept was clearly expounded by Krebs and Kornberg in 1957. ... 

The movement of ions through the membrane depends on two factors integrated in the concept of electrochemical gradient (1) the concentration gradient on both sides of the membrane (2) the electrical charge of the ion itself and the potential difference on both sides of the membrane. 

The term electromembrane process is used to describe an entire family of processes that can be quite different in their basic concept and their application. However, they are all based on the same principle, which is the coupling of mass transport with an electrical current through an ion permselective membrane. Electromembrane processes can conveniently be divided into three types (1) Electromembrane separation processes that are used to remove ionic components such as salts or acids and bases from electrolyte solutions due to an externally applied electrical potential gradient. (2) Electromembrane synthesis processes that are used to produce certain compounds such as NaOH, and Cl2 from NaCL due to an externally applied electrical potential and an electrochemical electrode reaction. (3) Eletectromembrane energy conversion processes that are to convert chemical into electrical energy, as in the H2/02 fuel cell. 

In these studies, a modulation of the transport rate is imposed upon a steady-state rate. As noted in Section 10.3, the ideas can be generalised through the concept of a transfer function linking fluctuations in current to fluctuations in the velocity gradient normal to the electrode. There are two distinct themes in the literature one is to impose a flow with known fluctuation characteristics in order to deduce information about electrochemical processes occurring at or near the interface, this being the focus of the present review the other is to use the variations in limiting current to deduce the characteristics of the flow, with an emphasis on analysis of the fluctuations in current to deduce characteristics of turbulent flow [81-85]. 

Nernst-Planck equation — This equation describes the flux of charged particles by diffusion and electrostatic forces. When the ion with charge ze is distributed at concentration c in the potential, cp, it has a one-dimensional flux of the ion, / = -Ddc/dx - (zF/RT) Dcdcp/dx [i]. This can be derived from the concept that the force caused by the gradient of the electrochemical potential is balanced with frictional force by viscosity, t], of the medium. When a spherical ion with radius ro is in the inner potential, cp, the gradient of the electrochemical potential per ion is given by... 

Concepts of local equilibrium and charged particle motion under - electrochemical potential gradients, and the description of high-temperature -> corrosion processes, - ambipolar conductivity, and diffusion-controlled reactions (see also -> chemical potential, -> Wagner equation, -> Wagner factor, and - Wagner enhancement factor). 

Formally, it will be even necessary to make corrections already in the starting flux equations. The detailed formulation of linear irreversible thermodynamics also includes coupling terms (cross terms) obeying the Onsager reciprocity relation. They take into account that the flux of a defect k may also depend on the gradient of the electrochemical potential of other defects. This concept has been worked out, in particular, for the case of the ambipolar transport of ions and electrons.230... 

Since the effect of TSH on TPMP+ uptake precedes the effect of the hormone on adenylate cyclase activity,1 7 the possibility exists that a primary mode of action of each of these effectors is to alter electrochemical ion gradients across the cell membrane. This concept is attractive in that it would serve to explain certain other findings. Thus, hCG causes changes in adrenal cell ion transport which not only precede adenylate cyclase stimulation, but occur at concentrations of the effector which have minimal effects on cyclase activity 1 9 cholera toxin and its B protein induce alterations in the permeability of liposomes reconstituted with "receptor" ganglioside in the absence of adenylate cyclase.50 51... 

We saw above that the concentration gradient at an electrode will be linear with respect to the spatial coordinate perpendicular to the electrode surface if the anode/cathode cell were operated at a constant current density and if the fluid velocity were zero. In actuality, there will always be some bulk liquid electrolyte stirring during current flow, either an imposed forced convection velocity or a natural convection fluid motion due to changes in the reacting species concentration and fluid density near the electrode surface. In electrochemical systems with fluid flow, the mass transfer and hydrodynamic fluid flow equations are coupled and the solution of the relevant differential equations is often a formidable task, involving complex mathematical and/or numerical solution techniques. The concept of a stagnant diffusion layer or Nemst layer parallel and adjacent to the electrode surface is often used to simplify the analysis of convective mass transfer in... 

According to the chemiosmotic concept therefore the central intermediate between the energy-transducing processes in the cytoplasmic membrane is the electrochemical potential gradient of protons, pmf-producing and pmf-consuming processes found in bacteria are listed below. 

---